The invention relates to isolation barriers for selectively isolating electrical circuits from each other. Such isolation barriers find use in modems and other devices, particularly those that require electrical isolation barriers between the devices and the public telephone network.
The Federal Communications Commission Part 68 has mandated that electrical connections to the public telephone network provide an isolation barrier between circuitry directly connected to that network (called the “line side” circuitry) and circuitry, such as a modem, that is directly connected to residential power (called the “system side” circuitry.) This isolation barrier must provide isolation such that a large magnitude voltage source of one thousand volts or fifteen hundred volts at 50 Hz or 60 Hz rms applied between various points on the device causes no more than 10 milliamperes leakage current.
The theory of isolation barriers is well known in the prior art. For example, U.S. Pat. No. 6,137,827 describes the theory and background of isolation barriers in great detail, incorporating by reference many patents illustrating isolation barriers and the devices that use them. Typically, electrical isolation is provided in the Data Access Arrangement (DAA) of the device. U.S. Pat. No. 6,137,827 and the patents incorporated therein by reference are each also incorporated herein by reference.
A conventional modem uses a voice band transformer in its DAA to provide The electrical isolation barrier. The transformer carries both the transmit signal and the receive signal. The separation of these two signals is done using a hybrid circuit of a type used to couple four-wire to two-wire circuits. Hybrid circuits are well known in the art and have four sets of terminals arranged in two pairs designed to produce high loss between two sets of terminals of a pair when the terminals of the other pair are suitably terminated. A modem using a voice band transformer DAA is known for its high reliability. However, the voice band transformer must handle low frequency (LF) signals from around 100-4000 Hz.
The primary source of distortion in a transformer is non-linearity, which causes signal harmonics. For voice band signals, many of these harmonics fall within this same 100-4000 Hz band. Signal harmonics are characterized by unwanted energy at multiples of the desired signal frequencies. So, the signal frequency component at 500 Hz will cause noise at 1000 Hz, 1500 Hz, 2000 Hz, 2500 Hz, 3000 Hz, 3500 Hz, and 4000 Hz; all within the desired signal band. On the other hand, if the signal is modulated to a high frequency, say 1 Mhz, then the desired signal will be in the range of 0.996 MHz to 1.004 MHz. Now, the 500 Hz component is at 0.9995 Mhz and 1.0005 Mhz. The lowest harmonic is at 1.999 MHz, well above the highest signal frequency of 1.004 MHz. Therefore, a simple low-pass filter can be used to remove the harmonics, and no distortion will be caused in the desired signal. The linearity of the transformer is largely dependent on the magnetic inductance density in its magnetic core. The higher the magnetic inductance density, the less linear the transformer and the higher the energy of the signal harmonics. Therefore, a high linearity requirement is placed on the voice band transformer, which generally increases its size and cost.
In addition, a modem DAA using a voice band transformer typically uses a direct driver approach. In this approach, the transmitted signal from the modem driver proceeds through the transformer directly without further amplification. This direct driver approach requires the transformer to deliver high transmit power, further increasing the linearity requirement of the transformer. Because of these drawbacks a satisfactory voice band transformer for this type of electrical isolation is bulky and expensive.
There are several approaches to solve this problem. One approach is to use digital transformers or pulse transformers to replace the voice band transformer. However, a digital or pulse transformer is binary and therefore cannot carry two signals, the transmit signal and the receive signal, simultaneously. To allow both the transmit signal and the receive signal to be transmitted acceptably, one has to either use two pulse transformers, one for the transmit signal and the other for the receive signal, or resort to some sort of time division multiplexing method to carry the transmit signal and the receive signal alternately. However, this time division multiplexing method will destroy the self-clock ability of the signal. Therefore the clock signal has to be carried by a different means, typically using another transformer and adding further expense to the product. Another disadvantage of digital or pulse transformers is that they must be operated in their saturation range, requiring more power than similar transformers operated in their non-saturated (or “linear”) range.
Another approach is to use capacitive coupling. This approach uses one or more high voltage capacitors as The electrical isolation barriers because a capacitor typically exhibits good linearity. Therefore, separation of the transmit signal and the receive signal using a hybrid circuit is possible. On the other hand, a LF voice band signal, required in modems, requires a large capacitor. Such a high voltage large capacitor is expensive. Therefore, some means are used to modulate the signal to a higher frequency to reduce the capacitor requirement.
Yet another approach is to use high voltage optical couplers. Again, due to the typically highly non-linear property of this type of optical device, separate couplers must be used for the transmit signal and the receive signal. In a DAA using optical coupling as The electrical isolation barrier, one can use a base band approach, or pass band approach. In the base band approach, the voice band signal is transmitted directly through the optical couplers. However, this has the drawback of requiring an elegant method to compensate for the non-linearity of the optical couplers. In the pass band approach, some means are used to modulate the signal to a higher frequency to reduce the impact of the non-linearity of the optical couplers, adding additional cost and complexity to the solution.
In addition to the transmit and receive voice band signals that modems of the type under discussion must transmit, there are control and status signals that also need to pass through The electrical isolation barrier. These latter signals are either carried through a separate isolation barrier, or multiplexed with the voice band signal and carried over the same isolation barrier.